In several previous terrestrial systems, very high levels of solar concentration (hundreds of suns) have been achieved with reflective optics to generate steam for powering turbines and direct conversion of heat to electricity via Sterling engines or photovoltaic (PV) cell for direct electrical conversion. Refractive optics in many forms, mainly Fresnel lenses, has been used for moderate levels of concentration (e.g., 20 suns) using flat or curved lenses to focus on a linear PV receiver. Flat or domed Fresnel lenses can achieve hundreds of suns of concentration using spot PV receivers.
MIDWAY Corporation, Chicago, Ill., for example, marketed a flat aperture configuration, with multiple Fresnel lenses concentrating light on small PV cells (1-2 cm2) distributed on the back plane of the structure with a focal length less than a foot. The heat was dissipated in the sheet metal back plane, and was adequate as long as the small receivers were adequately spread apart.
ENTECH Corporation, Keller, Tex., has developed both trough (curve lens) line concentrators and domed lenses for spot concentrators, and has flown a domed Fresnel lens in space using dual-junction cells. In the case of the line array, the terrestrial system uses passive cooling (aluminum back plane with extruded aluminum fins) for 20× concentration. In space application, ENTECH uses high thermal conductivity graphite for heat spreading for both domed and trough concentrators. In each case, however, the size of the PV receiver is relatively small, with a short focal length lens.
As long as the lenses and receivers are small, the thermal management problem can be solved through passive cooling techniques. But, if the size of the system (larger lenses, receivers, and focal lengths) is increased to achieve “economies of scale,” the thermal management of waste heat (that not converted to electrical energy by the PV conversion) becomes technically challenging.
Traditionally, the cooling of high power electronics requires active cooling to remove kilowatts of heat. This often entails pumping of water or other working fluids through cold plates, or in the extreme, the use of refrigeration units to chill the working fluid. In traditional two-phase systems, the use of compressors and heat exchangers is required to remove the heat. These often entail large investment of equipment, require additional kilowatt of electrical energy to operate refrigeration equipment, all of which adds considerably to the cost, and weighs against “scaling up” the high concentration (hundreds of suns) PV systems.
Needs continue to exist for improved solar power methods and apparatus, and for assembly methods that allow rapid deployment of arrays of any kind.